956 PHYSIOLOGY 



to get summation of contractions by putting in a stimulus, however strong, 

 before the muscle has returned to rest. If, however, the propagation of the 

 first contraction throughout the heart-muscle be retarded or prevented by a 

 partial death of the tissue, or by stimulus of the vagus nerve, it is possible, 

 as Frank has shown, to obtain an apparent summation of two stimuli, i.e. 

 a curve in which the second contraction is superposed on and rises higher 

 than the first. Such a result, on the explanation given above, would be due 

 to a phenomenon of ' block ' limiting the propagation of the first contractile 

 wave, and yielding more to the secondj though this is not the explanation 

 given by the original observer. 



SUMMATION OF STIMULI 



If an isolated frog's ventricle which is not beating be stimulated with 

 inadequate shocks, it may be found, on repeating these shocks at short 

 intervals of time, that they become adequate and cause a contraction of the 

 ventricle. K stimulus therefore which is subminimal 

 may nevertheless cause some change in the heart- 

 muscle, so that the latter responds more readily to 

 L subsequent stimuli. 

 FIG. 436. Group of pul- A similar improving effect of previous stimulation 

 cfs^ch S a3.' ^ on the condition of the heart-muscle may be observed 

 on the contractions themselves. Thus in a Stannius 



preparation, if the ventricle be excited with single induction shocks, once 

 in every ten seconds, the first four or five contractions form an ascending 

 series, each contraction being rather higher than the preceding one. This 

 is often spoken of as the ' staircase phenomenon ' (Fig. 436). 



THE REFRACTORY PERIOD 



At each contraction of the heart-muscle there is a sudden decomposition 

 of contractile material which, so far at least as concerns the incidence of an 

 external stimulus, is maximal, i.e. complete. Directly this has occurred a 

 process of assimilation or re-formation of contractile material begins. This lasts 

 throughout the diastolic period, and the store of contractile material is at its 

 maximum just before the next contraction. A mechanical analogy is furnished 

 by a bucket into which a stream of water is constantly flowing, and which tips 

 up automatically and empties out its contents as soon as the water reaches 

 a certain height. It is evident that the power of the heart-muscle to contract 

 in response to a stimulus (its ' irritability ') must be at a minimum imme- 

 diately after the automatic discharge or decomposition has taken place, and 

 will continually increase from this point as the store of contractile material 

 grows, until it arrives at such a height that the explosive discharge occurs 

 spontaneously. Hence in each cardiac cycle there is a period, known as the 

 refractory period, in which stimuli applied to the heart have no effect. This 

 will be followed by a period in which a stimulus is followed by an extra 

 contraction, but with a prolonged latent period. Just before the next 

 spontaneous contraction the irritability is at its height, and the heart-muscle 



